73. Expando

Before getting stuck into formal object definitions, let’s take a look at the Expando class that ships with Groovy. Expando provides a very flexible way to construct an object on the fly.

In the example below the monster variable is dynamically assigned a set of fields and methods (through the use of closures).

def monster = new Expando()

monster.with {
    //Let's add some properties
    name = 'Dr Frankensteins Monster'
    owner = 'Dr Frankenstein'
    height = 8

    //Add some closures (methods)
    getOwner = { owner }
    getName = { name }
    getHeight = { height }

    scareVillage = { village ->
        println "Look out $village here comes $name!"
    }
}

monster.scareVillage('Darmstadt')

The monster Expando looks to be acting in a manner similar to a Map but it gives us more functionality. Whilst the properties can be assigned to an Expando instance in the same way as a Map, the addition of a closure illustrates the difference. In the example below the use of a Map to define the vampire variable demonstrates that the scareVillage can’t rely on an instance field (name) so must refer back to the vampire map:

def vampire = [ : ]
vampire.name = 'Dracula'

vampire.scareVillage = { village ->
    println "Look out $village here comes $vampire.name!"
}

vampire.scareVillage('London')

By using the Expando class instead of a Map, the instance fields can be accessed.

Using Expando with CSV data

Expando can be useful when we want to consume data from a source (such as a file) and manipulate it as an object. One such example is an application that reads a comma-separated file (a CSV) that contains a header row and multiple subsequent data rows. The header row tells us the fields in our data objects and we can start to throw in some methods to help handle our data.

I’ll start with a full listing of a script that accepts CSV data (I’ll just use a String but it could be from a file) about various contacts. Expando will help me then produce a vCard for each contact. Take a read of the full listing and we’ll then break it down into easier chunks.

//This is a basic CSV (Comma-separated file)  described as a string
def table = '''\
	name,email,phone
	Fred,fred@email.com,555-678345
	Alex,alex@email.com,555-987123
	Simon,simon@example.com,555-567321
	'''

//We define the separator as the comma character
def separator = ','

//Read the CSV in and break it up by linebreaks
def csv = table.tokenize()

//Extract the first row - it contains the headers (field names)
header = csv[0].tokenize(separator)
csv.remove(0)

def contactList = [ ]

for (row in csv) {

    Expando contact = new Expando()

    //Now setup the contact class with field names based on the header
    row.tokenize(separator).eachWithIndex { key, value ->
        contact.setProperty header[value], key
    }

    contact.getVCard = {
        """BEGIN:VCARD
            VERSION:4.0
            N:$name
            EMAIL:$email
            TEL;TYPE=work,voice;VALUE=uri:tel:$phone
            END:VCARD"""
    }
    contactList << contact
}

contactList.each {
    println it.vcard
}

The handy tokenize method is used to break the CSV data into individual rows. The first row is actually a header row and we perform another tokenize to extract each header item (field) and then remove the header row from the data table:

def csv = table.tokenize()
header = csv[0].tokenize(',')
csv.remove(0)

A contactList is then defined - this will hold our list of contacts:

def contactList = []

The code now iterates through the data table. For each row, an instance of Expando is declared:

Expando contact = new Expando()

tokenize is again used to separate each field in the row. The eachWithIndex method passes the field’s value and its index into the closure’s key and value parameters respectively. I use the index to work out the field name in header and set the value as the field from the row:

//Now setup the contact class with field names based on the header
row.tokenize(',').eachWithIndex { key, value ->
    contact.setProperty header[value], key
}

The getVCard closure is then added to the contact - this will construct a vCard for use in other systems:

contact.getVCard = {
    """BEGIN:VCARD
    VERSION:4.0
    N:$name
    EMAIL:$email
    TEL;TYPE=work,voice;VALUE=uri:tel:$phone
    END:VCARD"""
}
contactList << contact

Once each row has been consumed we can display our contactList:

for (contact in contactList) {
    println contact.getVCard()
}